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Essay: Spintronics Nirav Parekh _________________________________________________________ ____________ ABSTRACT The review paper describes a new era of devices based on spintronics. Spintronics devices exploit the electron’s spin or magnetic moment to perform their functions. Unlike conventional charge based semiconductor electronic devices, which works on charge injection, transport, and controlled manipulation, spintronics device specifically exploits spin properties. These properties are exploited by adding the spin degree of freedom to conventional charge based electronic devices or using spin alone to yield potential advantage of non-volatility, increased data processing speed, decreased power consumption, and increased integration densities. 1

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Page 1: Nirav spintronics 1

Essay: Spintronics Nirav Parekh_____________________________________________________________________

ABSTRACT

The review paper describes a new era of devices based on spintronics. Spintronics devices

exploit the electron’s spin or magnetic moment to perform their functions. Unlike

conventional charge based semiconductor electronic devices, which works on charge

injection, transport, and controlled manipulation, spintronics device specifically exploits spin

properties.  These properties are exploited by adding the spin degree of freedom to

conventional charge based electronic devices or using spin alone to yield potential advantage

of non-volatility, increased data processing speed, decreased power consumption, and

increased integration densities.

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Essay: Spintronics Nirav Parekh_____________________________________________________________________

INDEXPage no.

1. Introduction 3

2. Spin Based Devices 5

2.1 Giant magnetoresistance (GMR) devices 5

2.2 Spin transistor 6

3. Manipulation of Electron Spin 8

3.1 Generation of spin polarization 8

3.2 Spin injection and spin-polarized transport 8

3.3 Spin detection 11

4. Spin Relaxation 12

5. Conclusion 14

References 15

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Essay: Spintronics Nirav Parekh_____________________________________________________________________

1. Introduction

Spintronics is a new paradigm in electronics [6]. It is based on exploitation of spin, a quantum

property of electron. Therefore, it is called spintonics. Control of electrical properties and

modification of information, by spin manipulation, are the two main goals of this field.

There are total three categories of spintronics based devices: 1) ferromagnetic metallic alloy

based devices, 2) semiconductor based devices and 3) the devices that manipulate the

quantum spin states of individual electrons for information processing [8].

Ferromagnetic metallic alloy based devices are mainly used in memory and information

storage. They are also termed as magnetoelectronics devices [8]. They rely on the giant

magnetoresistance (GMR) or tunnelling magnetoresistance effect. Magnetic interaction is

well understood in this category of devices [5].

Semiconductor spintronics devices combine advantages of semiconductor with the concept of

magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET.

To make semiconductor based spintronic devices, researchers need to address several

following different problems. A first problem is creation of inhomogeneous spin distribution.

It is called spin-polarisation or spin injection. Spin-polarised current is the primary

requirement to make semiconductor spintronics based devices. It is also very fragile state.

[14]. Therefore, the second problem is achieving transport of spin-polarised electrons

maintaining their spin-orientation [5]. Final problem, related to application, is relaxation time.

This problem is even more important for the last category devices [8]. Spin comes to

equilibrium by the phenomenon called spin relaxation. It is important to create long relaxation

time for effective spin manipulation, which will allow additional spin degree of freedom to

spintronics devices with the electron charge [3]. Utilizing spin degree of freedom alone or add

it to mainstream electronics will significantly improve the performance with higher

capabilities [6].

The third category devices are being considered for building quantum computers. Quantum

information processing and quantum computation is the most ambitious goal of spintronics

research. The spins of electrons and nuclei are the perfect candidates for quantum bits or

qubits. Therefore, electron spin and nuclear based hardwares are some of the main candidates

being considered for quantum computers [15].

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Essay: Spintronics Nirav Parekh_____________________________________________________________________

Spintronics based devices offers several advantages over conventional charge based devices.

Since magnetised materials maintain their spin even without power, spintronics based devices

could be the basis of non-volatile memory device. Energy efficiency is another virtue of these

devices as spin can be manipulated by low-power external magnetic field. Miniaturisation is

also another advantage because spintronics can be coupled with conventional semiconductor

and optoelectronic devices.

However, temperature is still a major bottleneck. Practical application of spintronics needs

room-temperature ferromagnet in semiconductors. Making such materials represents a

substantial challenge for materials scientists [16].

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2. Spin based Devices

The present status of spintronics devices at the commercial level is limited to giant

magnetoresistance (GMR) based devices. In GMR based memory devises electron spin play

passive role [12]. It is limited to detect the change of magnitude of resistance depending on

direction of the spin [12]. The change in resistance is controlled by a local or an external

magnetic field [2, 12]. But, it is predicted that spintronics can go beyond this passive spin

device by integrating electron spin into conventional semiconductors. Thus, the technology

based on spintronics may replace conventional semi-conducting devices by introducing active

control of electron spin [12].

2.1 Giant Magnetoresistance (GMR) devices

The read heads in modern hard drives and non-volatile, magnetic random access memory

(MRAM) are the two application of GMR effect.

In 1988, Albert Fert’s group discovered GMR effect. They observed that when multi layers of

alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic

field, they exhibit large change in electric resistance, which also known as magnetoresistance

[2].

Figure 1 Giant magneto resistance effect; (a) electron transport takes place when magnetization

direction of both ferromagnetic regions aligned parallel to each other, (b) electrons are facing

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high resistance and scattered away near interface when magnetization direction of both

ferromagnetic regions are opposite to each other (b)[8].

The change in resistance depends on the relative orientation of the magnetization in magnetic

layers [3]. The resistance to passage of current is low when the ferromagnetic layers align in

the same direction and transfer of current takes place dynamically (fig 1 (a)). If they align

themselves in opposite directions electrons scattering occurs near interface and a high

resistance path is produced [2] (fig 1 (b)). The relative orientation of magnetic layers can be

altered by the applying external magnetic field [2]. This effect is called spin-valve effect [2].

These multi layers are used to configure the GMR devices.

The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are

stored as the minute magnetic areas on the surface of HDD [2]. ‘Zero’ is stored, when the

magnetic layers align themselves in one direction and ‘one’ when they align in opposite

directions [2]. The read head reads the data by sensing a change in voltage corresponding to a

change in resistance [2]. It reads 1 when resistance is higher and 0 when resistance is lower

[2]. Thus, the ability of read head to sense minute changes in voltage corresponding to small

changes in magnetic fields will allow data storage at highest packing densities in small

magnetic particles [2]. The expected value of storage densities may reach to 100 gigbites per

square inch by using synthetic Ferromagnets [6]. There are three types of GMR.

2. Spin transistors

The spin-transistors exploit electron spin either by spin-valve effect or by active control of

electron spin [2]. The design of transistor is similar to that of GMR devices. It consists of

three layers, out of which the non-magnetic layer is sandwiched between the two

ferromagnetic layers [2].

Johnson was the first to propose about spin-valve transistor. As per him, the first magnetic

layer acts as a spin injector or emitter while the second acts as a spin detector or collector [2].

The non-magnetic layer acts as a base [2]. The magnetization direction of the collector can be

changed by the application of an external magnetic field [2]. When the voltage is applied

across the emitter-base, it generate electrons with either spin-up or spin-down [2]. When the

magnetization direction of emitter and collector is parallel, the current can flow throw the

base to the collector [2]. The electrons face high resistance when the relative magnetization

direction is opposite. Thus, device acts as one-way switch [2]. Electron spin plays passive role

in Johnson’s spin-valve transistor.

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Figure 3 Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in

transport channel (a) it enables current flow from source to drain. With applied gate voltage,

electrons change their spin state from parallel to anti parallel to the direction of magnetization of

ferromagnetic layer (b) this offers high resistance to flow of current. Therefore, electron

scattering occurs at drain and no current flow from source to drain [8].

The first model of transistor using active control of electron spin was proposed by Datta and

Das. In the Datta-Das field effect transistor, the non-magnetic layer acts as a gate while two

ferromagnetic layers act as source and drain respectively (fig 2(a)) [4]. The gate plays an

important role in Datta-Das field effect transistor. The gate modifies electron spin by

generating effective magnetic field and thereby in switching the transistor [4]. When voltage

is applied to the gate, it generates effective magnetic field (fig 2(b)). Thus, by modifying gate

voltage one can modify electron spin [4]. The electrons ballistically transport in transport

channel, if its spin is parallel to the magnetization direction of drain (spin detector) [4].

Otherwise, it will scattered away [4].

The control of charge current in spin transistor is similar to the conventional transistors [2, 4].

But, the spin transistors possess advantage over conventional transistors. They are smaller in

size, and consume less power [2]. Still, the spin-transistors are exist is in prototype models

because of theoretical limitation related to spin behavior in different materials.

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3. Manipulation of Electron Spin

Spintronic devices are based on careful manipulation of the electron spin. The spin can be

easily manipulated by applying external magnetic field or by shining polarized light [12]. In

general, the scheme of spin manipulation works fundamentally on: (1) generation of spin-

polarized electron, (2) injection and transportation of the spin-polarized electron, and (3)

detection of the spin-polarized carriers with information.

3. 1 Generation of spin polarization

The generations of spin-polarized electron spins mean generation of spin polarized current.

This spin polarized current carries non-equilibrium spin population. The Spintronics devices

detect the distribution of spin-up and spin-down electrons in spin polarized current to control

the current [1]. This phenomenon of controlling current in spintronic device makes it suitable

to act as electronic switch of transistor. Thus, the control of current is then either a control of

phase of electron spin or spin-population. It can be generated by transport, optical, and

resonance methods or by their combination [4]. Figure 2 shows the schematic representation

of generation of spin-polarized current by transport method.

3. 2 Spin injection and spin-polarized transport

The spintronic device requires efficient transport of generated non-equilibrium spin (spin-

polarized current) across the electrode/sample interface. The transport of non-equilibrium spin

across interface is called spin injection. The non-equilibrium spin can be injected by driving

ordinary current through ferromagnetic electrode to sample. The current can be driven in

plane plan of interface called 'current in plane (CIP) geometry' (fig 3(a)) or perpendicular to

the interface called 'current perpendicular to plane (CIP) geometry' (fig 3(b)). The spin can be

also injected by optical method. The efficiency of spin injection is determined by rate of

accumulation of non-equilibrium spin in sample. There are several proposed ways to transport

spin-polarized current across interface. These are: (1) formation of Ohmic contact between

electrode-sample interface, (2) Ballistic electron injection, (3) electron tunneling from space

charge region and, (4) Hot spin injection [4, 6].

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Figure 3 current flow across interfaces; current flow in the plane geometry (CIP) (a), current flow perpendicular to the plane geometry (CPP) (b) [4]

Figure 4 Spin injection in non ferromagnetic region via ferromagnetic region; equivalent circuit diagram for ferromagnet/non-ferromagnet interface (a) accumulation of nonequilibrium spin at non-ferromagnetic region (b) non-equilibrium spin state in non-ferromagnetic region (c) [4].

Essay: Spintronics Nirav Parekh_____________________________________________________________________

Ohmic injection

The most basic approach to spin injection is through the perfect Ohmic contact between

ferromagnetic/non-magnetic (F/N) interfaces (fig 4 (a)) [4, 6]. The interface can be produce

by taking metals or semiconductors or superconductors as non-magnetic region with

ferromeget. The degree of spin injection in non-magnetic region depends on the ratio of the

conductivities of ferromagnetic region (F) and non-magnetic region (N) [6]. For typical

conductivity mismatch, when conductivity of F region ≤ N region, higher the spin injection

efficiency (fig 4(b) and (c)). When conductivity of F region ≥ non-magnetic region, smaller

the spin injection efficiency. This phenomenon is called “conductivity mismatch” [4, 6]. In

the case of ferromagnet/semiconductor interface, Ohmic contacts resulted from the doping of

semiconductor surface. However, doping leads to loss of spin polarization by spin-flip

scattering [4, 6]. The electrochemical potential of N region increases with spin injection. The

difference of spin dependent electrochemical potentials generates effective resistance δR on

either side of F/N interface. In superconductor/F interface, increase in total resistance with

spin injection results in switching superconducting state to normal state of much higher

resistance. [4]

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Ballistic electron injection

The ballistic spin injection works on principle of GMR effect and electrons are dynamically

transported. The ballistic transport is favorable in ferromagnet/non-magnet/ferromagnet

(F/N/F) interfaces. The F/N/F interface if formed by sandwiching a non- ferromagnetic layer

of finite thickness between two finite ferromagnetic layers [4]. Fully ballistic transport takes

place when ferromagnetic layers aligned in the same direction [7]. This condition provides

low resistance path to the spin polarize current. The probability of spin polarized electron

back flow or reflection is less in ballistic transport, once it enters in the non-magnetic region.

The transmission probability of ballistic transport depends on difference of two spin

conduction sub bands of the ferromagnet and the conduction band of the semiconductor [4,

6].

Tunneling spin injection via Schottky barrier (F/S interface)

The interface made up of GaAS (p-region) and ferromagnet (n-region) acts as a typical

Schottky barrier [4, 9]. Properties of Schottky barrier strongly depend on bias and doping of

semiconductor. There is no spin injection at small forward biases due to formation of

depletion region near interface [4]. The depletion region offers resistance to flow of current

[4, 9]. The spin injection is possible at large forward bias because of electric drift lead by non-

equilibrium spin already present in the n-region [4]. The results are more promising in the

case of F (p-region)/S (n-region) in reverse bias. The barrier acts as purely tunneling barrier in

reverse bias due to extraction of spin from non-magnetic region [4, 9]. Thus, tunneling

junctions are considered to be most probable candidate for enhanced spin injection [6].

Hot electron injection

An electron with energy higher than Fermi level is called ‘Hot electron’ [6]. These hot

electrons are injected in ferromagnetic region by tunneling. The transmission probability of

electrons depends on band structure of the F/N interface. As high as 90% efficiency will be

possible this method, when spin-flip scattering at the interface is very less. The disadvantage

of this method is lower overall efficiency [6].

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3.3 Spin detection

Spin detection typically depends on the sensing the changes in the signal due to spin injection

[4]. The injection of non-equilibrium spin either induces voltage or changes resistance

corresponding to buildup of the non-equilibrium spin [4, 13]. This voltage can be measured in

terms of change in resistance by potentiometric method; while change in resistance can be

measured in terms of voltage by balancing Wheastone Bridge [4, 13]. The transport and

optical methods of spin detection are most widely adopted to detect spin. The efficiency of

spin detection in transport method is depends on interface properties. Therefore, spin

detection is low and also suffered from difficulty discuss above [4, 6]. The optical spin

detection technique is well established. The spin can be detected by determining the helicity

of emitted light from LEDs connected with interface [4].

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4. Spin Relaxation

Non-equilibrium spin accumulates in non-magnetic region due to process of spin injection. It

comes to equilibrium by the phenomenon called spin relaxation [4]. The rate of accumulation

of non-equilibrium spin depends on the spin relaxation [4]. Electrons can remember their spin

state for finite period of time before relaxing. That finite time period is called ‘Spin lifetime’

[4, 1]. Longer lifetime is more desirable for data communication application while shorter for

fast switching [4]. The distance traveled by the electron without loosing spin state is called

‘Spin diffusion length’ [4]. It is most important variable in spintronic devices, which

determines maximum allowable thickness of the non-magnetic region in device. It is also

depend on spin lifetime [4]. There are four proposed ways by which conduction electrons of

metals and semiconductors relax: (A) The Elliott-Yafet mechanism, (B) The D’yakonov-

Perel’ mechanism, (C) The Bir-Aronov-Pikus mechanism, and (D) hyperfine-interaction [4].

Elliot-Yafet Mechanism

Elliot (1954) first suggested that electron spin relaxation occurs via momentum scattering.

Momentum scattering occurs when lattice ions or photons bring on spin-orbital coupling in

the electron wave function [4]. This spin-orbital coupling introduces wave functions of

opposite spin [4]. Now, electron wave functions with related spin have an admixture of the

opposite spin state [4]. These combinations of spin-up and spin-down momentum lead to

relaxation of electron spin. The mechanism is dominant in small-gap semiconductors with

large spin-orbit splitting. [4]

D’yakonove-Perel’ Mechanism

This mechanism comes into play, when the systems lack inversion symmetry [4]. The

electrons feel an effective magnetic field, resulting from the lack of inversion symmetry, and

from spin-orbit interaction [4]. These fluctuating magnetic fields randomly change the

magnitude and direction of electron spin precession [4]. They also randomize the spin. This

spin randomization is more effective than momentum scattering [4]. Therefore, spin

dephasing occurs because of the momentum dependent spin precession along with momentum

scattering. This mechanism plays important role with increase in temperature and increase in

band gap. [4]

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Bir-Aronov-Pikus Mechanism

The holes also possess spin [10]. The spin of hole can be exchange with conduction electrons.

These exchanges proceed through scattering and lead to spin relaxation of conduction

electron in p-doped semiconductors (Bir, 1975) [4, 10]. Holes have shorter spin coherence

time and spin exchange between electrons and holes is very effective. Ultimately, it will leads

to spin decoherence. This mechanism is of importance at low temperatures. [4, 8]

Hyperfine-interaction Mechanism

Hyperfine-interaction comes from the magnetic interaction between the magnetic momentum

of nuclei and electrons. In semiconductor hetrostructures, this mechanism is responsible for

spin dephasing of localized or confined electron spins [4, 10].

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5. Conclusion

The new technology based on spintronics utilizes electron spin and charge in conventional

electronics [1, 4]. The spin can be effectively utilized by careful manipulation of electron spin

dynamics [1]. The effective manipulation adds additional spin degree of freedom to the

devices [3]. The potential advantage is considerable increase in capacity of conventional

electronic devices [1]. But, it suffers from fundamental limitations. The spin dynamics is not

clearly understood in transport across interface [12]. This uncertainty imposes limits on

design of devices [12]. However, in recent years, understanding of spin dynamics in metallic

multilayer gives partial success in utilizing electron spin as GMR read head and data storage

devices [6]. But, the projection of spintronics will go beyond this and may end regime of

charge based electronic [12].

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References

1. Robert Matthews “Take a spin” New Scientist, Feb. 28, 1998, P. 24-28.

2. Peter Rodgers “Giants in their field” New Scientist, Feb. 10, 1996, P. 34-37.

3. Stuart A. Wolf and Daryl Treger “Spintronics: A new paradigm for electronics for the

new millennium” IEEE transactions on magnetics, Vol, 36, No.5 September 2000.

4. S. Das Sharma, Jaroslav Fabian, Igor zutic “Spintronics: Fundamental and

applications” Reviews of modern Physics, Vol. 76, No. 2, April 2004.

5. M Oestreich, M Bender, JH¨ubner, D H¨agele,WWR¨uhle, Th Hartmann, P J Klar, W

Heimbrodt, M Lampalzer, K Volz and WStolz “Spin injection, Spin transport and

spin coherence” Semiconductor science and Technology 17 (2002) P. 285-297

6. S.A. Wolf, D. D. Awschalom, D. M. Treger “Spintronics: A spin-based electronics

vision for the future” Science Vol. 294, Nov. 2001.

7. Robert F. Service “Spintronics innovation bids to bolster bids” Science Vol. 297, July

5 2002.

8.  David D. Awschalom,. Flatte ME and Nitin Samarth “Spintronics” Scientific

American Jun 2002; 286(6): P. 66-73.

9. G Schmidt and L. W. Molenkamp “ Spin injection into semiconductors, physics and

experiments” Semiconductor Science and Technology 17 (2002) P. 310-321.

10. J. Strand, X. Lou, C. Adelmann, B.D. Schultz “Electron spin dynamics and hyperfine

interactions in Fe/Al0.1Ga0.9As/GaAs Spin injection hetrostructures”. Physics review B

72 (2005), 155308.

11. Dirk Grundler “Spintronics” Physics world April 2002.

12. S. Das Sharma, Jaroslav Fabian, Igor Zutic, Xuedong Hu “ Issues, Concepts, and

challenges in spintronics” Dept. of Phys, Uni. of Maryland.

13. Recardo L Gomez-Abal “Spintronics” PPT. Fritz-Haber Institute of The Max Plank

Society Berlin, Feb. 1, 2005.

14. Prof. Branislav K Nikolic “Spintronics: fundamental and application” University of

Jyväskylä.

15. http://www.physics.umd.edu/rgroups/spin/intro.html accessed on 8th may 2006

16. Hideo Ohno SEMICONDUCTORS: Enhanced: Toward Functional Spintronics:

Science 291: 840-841.

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